This application claims the priority of Korean Patent Application No. 2003-000778, filed on Jan. 7, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a magnetoresistive random access memory, and more particularly, to a magnetoresistive random access memory with high selectivity.
2. Description of the Related Art
A magnetoresistive random access memory (MRAM) is a spin electronic device that has been developed as a next generation memory device capable of replacing a conventional dynamic random access memory (DRAM) in which it has a fast data write speed but data stored inside is erased once an electric power is off, and a flash memory having a data write speed 1,000 times slower than the DRAM. The MRAM has a multilayer thin film structure of a ferromagnetic layer/an insulating layer/a ferromagnetic layer and stores data by controlling spin tunneling of electrons according to the magnetization direction of a magnetic material of the ferromagnetic layer.
FIG. 1 is a fundamental structural view of conventional MRAM cells. When an electric current is applied to both a bit line 11 and a word line 13, the magnetization direction of a free layer of a first cell 15 positioned at an intersection between the bit line 11 and the word line 13 is reversed. As a result, magnetic information is written on a magnetic memory bit. However, in an array of the MRAM, a magnetic field is unavoidably applied to second and third cells 17 and 19 that are respectively present on the word line 13 and the bit line 11 to which an electric current is applied.
FIG. 2 is a graph showing resistance-magnetic field (R-H) characteristics of the first cell 15 selected for data writing and the second and third cells 17 and 19 positioned around the first cell 15. Although the second and third cells 17 and 19 are not selected for data writing, a magnetic field is applied thereto.
f1 is a graph showing R-H characteristics of the first cell 15, f2 is a graph showing R-H characteristics of the third cell 19, and f3 is a graph showing R-H characteristics of the second cell 17. As can be seen from the f1, f2, and f3, there are two discrete states of R-H curves in conventional MRAM cells. Kinks, areas having resistance of an intermediate state to magnetic field, appear at points A and A′ of the f1, a point B of the f2, and a point C of the f3. The kinks are one of main factors that decrease selectivity of an MRAM array.
The first cell 15 has a switching field H0 lower than neighboring cells due to an electric field applied from the bit line 11 and the word line 13 that are perpendicular to each other. In the graph of the f1, a magnetic field H0 for full switching is achieved at a point of f1sw. When a switching field H0 of 28 Oe is applied, while the direction of a magnetic vector in a free layer of the first cell 15 is reversed, the direction of a magnetic vector in a free layer of the second cell 17 on the word line 13 is not reversed. Referring to the f3, a kink (the point C) is created at a magnetic field, Hc2, larger than the H0. Therefore, the magnetization reverse of the second cell 17 does not take place. However, referring to the f2, a kink (the point B) is created at a magnetic field smaller than the H0. Therefore, when the magnetic field of H0 is applied, the magnetic vector of some magnetic domains of the third cell 19 on the bit line 11 is partially reversed, thereby causing an error.
Therefore, in order to increase selectivity of a MRAM, it is required that a MRAM cell is newly designed toward the direction of having no kinks or having kinks incapable of adversely affecting selectivity.